Different working mechanisms exist for Photonic Integrated Circuit (PIC) or Planar Lightwave Circuit (PLC) optical switches. These include beam-steering, interference systems, such as directional couplers, and adiabatic systems, such as adiabatic couplers.
Beam-steering mechanisms have a limited scalability, are usually mechanically instable (sensitive to vibrations) and the losses are too high.
Interference based switches are strongly wavelength dependent. Due to the interference nature of this system, the conditions of cross and bar state strongly depend on wavelength, coupling coefficient (and thus fabrication tolerance on gap size and temperature) and effective index difference of the waveguides (and therefore on wavelength and fabrication tolerance of the width (and sensitivity on cladding index)).
This large sensitivity results in larger losses for a fabrication-design mismatch and wavelength-sensitivity which makes the design infeasible for different types of applications, e.g. applications that need a large operational bandwidth.
A third family of switches is based on adiabatic coupling or mode evolution coupling. Herein, as opposed to directional couplers, the goal is to keep the optical power always in one mode, i.e. the locally orthogonal mode (even in the transition from constituent modes to super modes) while the exact field-profile of the coupled modes changes over propagation distance. Light in mode 1 will remain in mode 1 while light in mode 2 remains in that same mode, but not necessarily in the same waveguide. No interfering optical parts are needed which therefore results in a switching mechanism which is not inherently sensitive to wavelength, temperature and fabrication. This is beneficial for the operational bandwidth of the switch and results in a lower sensitivity to manufacturing variability.
In U.S. Pat. No. 7,283,696 a directional coupler for switching is disclosed wherein a micro channel, formed in the substrate, is positioned in proximity of the core of a waveguide such that an amount of optical power extends into the portion of the micro channel in close proximity of the waveguide core and wherein a liquid can circulate in the micro channel. The micro channel is accurately (micrometer to sub-micrometer accuracy) aligned with the waveguides in the substrate. It is known by a person-skilled-in the art that the fabrication process to align fluidic channels with waveguides is a non-trivial manufacturing step.